1. Field of the Invention
This invention relates to electronic packaging and more particularly to high density, low cost packaging of electronic circuitry.
2. Description of the Prior Art
The prior art shows electronic packages where integrated circuit chips are mounted on thin, flexible sheets of insulating material. For example, see U.S. Pat. Nos. 3,780,352 and 3,781,596.
The prior art also teaches various techniques for avoiding problems caused by thermal mismatch between integrated circuit chips and the support on which the chips are mounted. One known technique for avoiding problems due to thermal mismatch is to mount the chip on a flexible film carrier. For example, U.S. Pat. No. 4,231,154 shows an integrated circuit chip mounted on a very thin layer of circuitized polyimide.
Double sided circuitry is also well known. Such circuitry generally includes a carrier with circuitry on both sides of the carrier. The circuitry on the two sides of the carrier is generally interconnected by vias or thru holes. The carrier in such circuitry is generally a material such as epoxy glass which can be etched or drilled to provide passages for the via interconnections.
None of the prior art teaches how to fabricate circuitry on both sides of a thin polyimide carrier where the circuitry on the two sides of the carrier is interconnected by vias which are etched before the polyimide is cured.
It is noted that in order to etch vias into polyimide one must have uncured polyimide. The polyimide that is generally available in sheet form is cured polyimide. The reason that only cured polyimide is generally available in sheet form is that the process normally used for making sheet polyimide cures the polyimide during the fabrication process. If one fabricates circuitry on both sides of a "cured" polyimide sheet, one cannot etch thru holes or vias into the polyimide sheet using conventional photoresist and etch techniques.